A number of prior art processes have been proposed for the preparation of oxalates and carbonates by oxidative carbonylation in the presence of alcohols including the use of metal salt catalysts, dehydrating agents and ferric or cupric redox agents in the solution.
The present invention is directed to an improved process for the preparation, in high yield, of oxalate esters, and exceeding the yield of carbonates. More particularly, it relates to the preparation of oxalates by reacting carbon monoxide with an alcohol under elevated temperature and pressure conditions in the presence of stoichiometric amount of palladium, rhodium, platinum, or copper salt catalyst and a stoichiometric amount of an amine base and includes the employment of iron (III) or copper (II) oxidant salts in addition to various counterions and ligands of the metal salt catalysts.
U.S. Pat. No. 3,114,762 discloses a method for the preparation of alkyl carbonates by reacting carbon monoxide with an alcohol in the presence of platinum or palladium chloride and in the added presence of an oxidizing salt for reoxidizing the catalyst in situ. The reaction is carried out at temperatures of from 20.degree. to 100.degree. C. and carbon monoxide pressures of 1 to 500 atmospheres. Runs carried out for comparison even at higher temperatures only resulted in trace amounts of the oxalate.
U.S. Pat. No. 3,393,136 describes a process for the preparation of oxalates by contacting carbon monoxide at superatmospheric pressure, with a saturated monohydric alcohol solution of a platinum group metal salt and a soluble ferric or cupric salt (redox agent) while maintaining the salts in a highly oxidized state by the simultaneous introduction of oxygen or the application of a direct current electrical potential to the reaction zone. When oxygen is employed, explosive mixtures of oxygen and combustible organic vapors in the gas phase must be avoided and water scavengers or dehydrating agents such as alkyl orthoformic acid esters must be added to the liquid phase to prevent the accumulation of water.
In a recent article by Donald M. Fenton and Paul J. Steinwand, Journal of Organic Chemistry, Vol. 39, No. 5, 1974, pp. 701-704, a general mechanism for the oxidative carbonylation of alcohols to yield dialkyl oxalates using a palladium redox system, oxygen and dehydrating agents has been proposed. In the absence of the necessary dehydrating agent, a large amount of carbon dioxide is formed and oxalates are not produced. The necessity of the iron or copper redox system during the oxalate synthesis is emphasized.
A recent West German Pat. No. 2,213,435 discloses a method for the synthesis of oxalic acid and oxalate esters in water and alcohol respectively. A platinum group metal salt, a salt of a metal more electropositive than the platinum group metal, e.g., copper (II) chloride and an alkali metal salt comprise the catalyst. Oxygen in stoichiometric amounts was employed as the oxidant. A disadvantage of such reaction is that explosive mixtures of oxygen and carbon monoxide are necessary to effect reacton. Under non-explosive conditions only trace amounts of oxalate can be obtained.
Many important commercial applications have been developed for the oxalate products of this invention, for example, as cellulose ether or ester and resin solvents, as dye intermediates and the preparation of pharmaceuticals.
The process of the present invention provides a high yield selectivity to the oxalate esters. Carbonate esters and carbon dioxide associated with such reactions are minimized by a critical regulation of the catalyst and oxidant anions and alternatively certain ligands and by maintaining the reaction mixture substantially anhydrous.
Other advantages of the present invention, as compared to known prior art processes are (1) elimination of hazardous operational conditions by avoiding explosive mixtures of oxygen and carbon monoxide, (2) avoiding any necessity for using dehydrating agents as no water is formed as a result of the instant oxidative carbonylation process; when air or O.sub.2 are used as the oxidant water and CO.sub.2 are formed and the presence of water always decreases the yield of oxalate and increases the CO.sub.2 ; (3) avoiding the use of large amounts of corrosive chloride ions, (4) ease of recovery and reoxidation of the metal salts in a stream of air or oxygen for reuse in the oxidative carbonylation process and (5) the ability to employ in the process as catalysts the more readily available copper salts in place of the more expensive platinum group metal salts.